A hydraulic brake booster communicates with a pressure source and is operable upon a brake application to provide a power assist during braking. An input member cooperates with a control valve to control communication of pressurized fluid to a pressure chamber and a piston within the brake booster is movable in response to pressurized fluid within the pressure chamber in order to generate a power assist for a brake application. The piston also defines a storage chamber located inside the piston and a charging valve communicates pressurized fluid from the pressure chamber to the storage chamber.
If the pressure of the pressurized fluid communicated to the pressure chamber is insufficient to move the piston during braking, an input member is engageable with the charging valve to open the storage chamber to the pressure chamber, thereby increasing the pressure of pressurized fluid within the pressure chamber.